An Experimental Setup to Study the Impact of Surface Disturbances on Boundary Layer Transition over a Transonic Laminar Wing Section

摘要

The design of an experimental setup that offers the ability to study the effect of surface imperfections (steps and gaps) on the pressure side of a transonic wing section is detailed. The motivation behind the special design was to duplicate the boundary layer characteristics of the reference transonic wing section over a flat plate in a low speed wind tunnel. An in-situ procedure, set up to analyze the stability behavior of the boundary layer based on the measured pressure distribution, revealed earlier boundary layer transition than expected. Possible explanations of this behavior were traced to two sources: an elevated turbulence intensity region in the wind tunnel test section, and to practical accuracy of setting certain step disturbances. Investigations into the problem of step inaccuracy showed inconclusive results. On the one hand, small spanwise step inaccuracies clearly generated turbulent spots. On the other hand, filling and sanding the step did not eliminate the intermittency. Despite the shortcomings of the experimental setup, which are described in detail, a set of test cases on forward- and backward facing steps with different step size were carried out. Similar to previous studies, backward facing steps are shown to be more detrimental to laminarity than forward facing steps.